The invention relates to the field of high-temperature superconductors and in particular to a new design of thin film based superconducting fault current limiters. It is based on a superconducting fault current limiter according to the preamble of claim 1.
High temperature superconductors may be applied in superconducting fault current limiters for electrical distribution or transmission networks. Such a current limiter owes its properties to the fact that at a sufficiently low temperature, a superconductor maintains its superconducting properties only as long as a current flowing through its body remains below a critical value. Said critical value is commonly known as critical current density (Jc) and basically depends on the temperature of the superconductor and a magnetic field in its interior.
In general, for applications of high temperature superconductors at high electrical powers the problems of the so-called xe2x80x9chot-spotsxe2x80x9d must not be neglected. Due to unavoidable inhomogeneities in the superconductor material or due to local thermal fluctuations the critical current density suffers small variations across the superconductor. Accordingly, in the case of a short circuit, the initial increase of the fault current will first exceed the critical current at the weakest point of the superconductor. At this point, the voltage drop begins to build up, and joule heating starts to set in, leading to a local increase in temperature and a local crash of superconductivity, i.e. the occurrence of a hot-spot. If the hot-spot does not spread quickly over the superconductor, the fault current will not be limited fast enough and the increase of heat at the hot spot location may finally lead to a destruction of the superconductor.
In electrical distribution or transmission networks including superconducting devices, in case of a fault, the voltage applied to the corresponding section of the network drops either deliberately (if the device is supposed to exhibit current limiting properties) or involuntarily along the superconductor. An ideal high temperature superconductor with a perfectly homogeneous critical current density jc and a uniform current distribution the latter will, in case of a fault, quench homogeneously over its entire length, i.e. warm over the critical temperature TC snd turn resistive. Accordingly, the voltage drops over the entire length of the superconductor, leading to small electrical fields and sub-critical energy densities.
A primary remedy is formed by a metallic electrical bypass, which is in close contact over the entire length of the superconductor and thus is electrically in parallel to every potential hot spot. The bypass offers an alternative current path, by means of which a fault current may circumvent the hot spot, thereby homogenising the voltage distribution.
In the German Patent Application DE 100 14 197.8 a superconductor arrangement for preferential use in a fault current limiter and comprising a track, band or wire of high-Tc superconductor material is disclosed. Weak spots with a reduced critical current IC are provided over the length of the superconductor track. Hence, in the event of a fault current, initial voltage drops start to develop at the weak spots. The dissipation produces heat which is propagating to adjacent regions of the superconductor. If the weak spots are close enough to each other, the superconductor quenches in a homogeneous way.
From the article (in the following referred to as ref.2) by M. Decroux et al., xe2x80x9cProperties of YBCO Films at High Current Densities: Fault Current Limiter Implicationsxe2x80x9d, IEEE Trans. On Applied Superconductivity, Vol.11, 2046 (2001), thin-film high temperature superconductors are known. A substrate serves a support for a thin epitaxial layer (thickness≈1 xcexcm) made from a ceramic high temperature material, in particular a composition according to the formula YBa2Cu3O7-x with a critical current density of 3xc3x97106 A/cm2 at a working temperature of 77 K. Extensive studies on these materials have shown the existence of a critical electric field Ec which determines the length of a dissipative portion of the superconductor upon application of voltage pulses of various heights.
It is therefore an object of the invention to create a thin film superconducting fault current limiter of the type mentioned above, which quenches in a controlled way and is not exposed to hot-spots in the case of a fault current occurring in a line comprising the current limiter. This object is achieved by a superconducting fault current limiter according to patent claim 1. Preferred embodiments are evident from the dependent patent claims.
A current limiter according to the invention is composed of several constrictions and interposed connecting sections. The constrictions or weak spots have a lower critical current and thus, in case of a fault current exceeding a nominal current, do revert to a resistive state in the first place, before any heating of the superconductor occurs. The total length and the cross section of the constrictions are such that the constrictions in their resistive state develop a resistance which is sufficient to build up a voltage drop that equals an applied voltage and to limit the current flowing through the constrictions to a value below the prospective xe2x80x9cunlimitedxe2x80x9d fault current. During this initial phase, the connecting sections remain superconducting, they start to turn resistive only at a later stage.
The present invention is based on the finding that a superconducting fault current limiter, after a voltage has been applied to it, develops different regimes as a function of time. In particular, after a transient regime, the superconductor follows an initial current source regime with a current essentially independent of the applied voltage and a current density equal to about 1.5 times the critical current density of the superconducting material.
The resistance of the constrictions during said initial regime is determined by the length of the constrictions and the resistivity averaged over the conductor cross section, which depends on the thicknesses of the superconducting film and an adjacent bypass layer. The resistivity of the constrictions may be chosen such as to limit the power density dissipated by the fault current in the constrictions to an acceptable value. By adjusting the resistance of the connecting sections, the dissipated power density at long terms, i.e. once the connecting sections have turned resistive as well, can be adapted independently.